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Identification of natural and synthetic diamonds by cathodoluminescence spectra
E. I. Lipatov, A. G. Burachenko, S. M. Avdeev, V. F. Tarasenko, M. A. Bublik
E. I. Lipatov, A. G. Burachenko, S. M. Avdeev, V. F. Tarasenko, M. A. Bublik, "Identification of natural and synthetic diamonds by cathodoluminescence spectra," Proc. SPIE 10614, International Conference on Atomic and Molecular Pulsed Lasers XIII, 106140Y (16 April 2018); doi: 10.1117/12.2302979 Event: XIII International Conference on Atomic and Molecular Pulsed Lasers, 2017, Tomsk, Russia Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 5/18/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
Identification of natural and synthetic diamonds by cathodoluminescence spectra E.I. Lipatov*a, A.G. Burachenkoa, S.M. Avdeeva, V.F. Tarasenkoa, M.A. Bublikb a Institute of High-current Electronics SB RAS, 2/3 Akadenichesky ave., Tomsk, Russian Federation, 634055; b All-Russian Research Institute of Automation named after Dukhov, 22, Suschevskaya st., Moscow, Russian Federation, 127055 ABSTRACT The cathodoluminescence spectra of nine diamond samples at temperatures of 82-295 K were investigated. According to the presence of the N3a vibronic system with the zero-phonon line at 2.68 eV in the luminescence spectra, six samples were identified as natural. By the presence in the luminescence spectra of the 2.56-eV vibronic system and the unstructured band at 2.54 eV, associated with nickel, two samples were identified as synthetic, grown at high pressure and high temperature. Due to exciton luminescence at 5.271 eV and the absence of any spectral features of impurity nature, one sample was identified as synthetic, grown by the chemical vapor deposition. Based on the data obtained, the technique for identifying of natural and synthetic diamonds has been proposed. Keywords: diamond, cathodoluminescence, identification, vibronic system, nitrogen aggregation
1. INTRODUCTION Currently, the main methods of diamond synthesis are high-pressure high-temperature growth (HPHT) [1] and chemical vapor deposition (CVD) [2]. The difference in the synthesis conditions causes the difference in the impurity-defective composition of diamond samples and, consequently, the difference in the optical absorption (OA) and photo/cathodoluminescence (PL/CL) spectra. OA and CL spectra of natural and synthetic diamonds contain information on the initial composition of the charge/gas mixture, synthesis conditions and subsequent radiation-temperature treatment. Progress on synthesis technologies provides optically pure diamonds of large size, and makes it actual the task of identification of synthetic diamonds and improved natural diamonds (i.e., subjected to special HPHT treatment) in the jewelry market. Previously, fianite (ZrO2) and moissonite (SiC) were used to simulate diamonds, which were easily identified by portable diamond testers measuring the thermal and electrical conductivity of the samples [3]. However, express identification of brilliants made from synthetic or improved diamonds requires other approaches, such as the Raman spectra [4], optical transmission [3, 5], PL and CR [3, 5] measurements. The most compact can be devices measuring the spectra of OA and PL/CL. In the present work, the CL spectra of nine natural and synthetic samples were studied. The analysis of spectral bands and electron-vibrational (vibronic) systems of optically active centers is carried out. Natural diamonds contained in the CL spectra the N3a vibronic system (VS) with the zero-phono line (ZPL) at 2.68 eV. The N3a center is a complex defect - the N3 center in the structure of "platelet". The conditions combination of N3a centers formation in natural diamonds and it duration leads to the difficulty of reproducing the N3a centers in synthetic diamonds. Observation of N3a VS in the luminescence spectrum of diamond sample makes it possible to classify it as natural. Synthetic samples did not show N3a VS in the CL spectra. HPHT samples demonstrated the 2.56-eV VS or the broad structureless band at 2.5 eV, which are associated with impurity-defective complexes containing nickel atoms. A high quality CVD sample demonstrated the recombination of free excitons at 5.271 eV.
*
[email protected]; phone +7 3822 491-685; www.hcei.tsc.ru
International Conference on Atomic and Molecular Pulsed Lasers XIII, edited by Victor F. Tarasenko, Andrey M. Kabanov, Proc. of SPIE Vol. 10614, 106140Y © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2302979 Proc. of SPIE Vol. 10614 106140Y-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 5/18/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use
2. EXPERIMENTAL In the work, the CL spectra of nine diamond samples (natural, synthetic HPHT and CVD) were measured. Their designations and main characteristics based on the results of visual inspection are given in Table 1. Table 1. The designations, dimensions, color and degree of transparency of diamond samples.
Sign CN1 CN3 CN4 CN5 CN8 CN9 CN11 C4 C5
Thickness, mm 0.39 0.195 0.3 0.375 0.3 0.46 0.83 0.25 0.5
Area, mm2 17.07 14.72 10.8 14.22 14.11 10.43 10.28 19.63 100
Color
Transparency
Form
White Yellow Yellow Green Green White Gray-green Colorless Colorless
Translucent Transparent Transparent Transparent Transparent Translucent Translucent Transparent Transparent
Rectangular Rectangular Rectangular Rectangular Rectangular Square Triangular Round Square
To excite CL in the samples, the SLEP-150 and GIN-55-01 generators with gas-filled diodes were used [6]. For the SLEP-150 generator, the CL excitation parameters were as follows: beam current density 50 A/cm2, beam pulse duration 100 ps, pulse repetition frequency 1 Hz. For the GIN-55-01 generator, the beam current density is 1.6 A/cm2, the beam pulse duration is 100 ps, the pulse repetition frequency is 60 Hz. In the visible range, the CL of the C5 sample was characterized by the lowest intensity. Therefore, for a reliable registration of the luminescence spectrum, the CL of the C5 sample was excited by the GIN-55-01 generator, whose frequency mode of operation allowed the signal to accumulate by the spectrometer. The CL of other samples was much more intensive, and the SLEP-150 generator was used. To measure the integrated CL spectra of samples, the experimental setup was used, which scheme is shown in Fig. 1. A pulsed electron beam from the generator 1 excited the CL in the sample 2. The sample was placed in the evacuated chamber 3 and was placed on the copper holder 4 on the hollow copper heat sink 5 that emerged into the vessel 6 with liquid nitrogen. The thermistor 7 (Pt1000, Heraeus) was attached to the copper holder 4. Its resistance was measured through vacuum-tight bushings 8 by the digital ohmmeter 9. The CL emission passed through the sample 2 and passed through the optical fiber 10 to the spectrometer 11. After signal accumulation, the spectra captured at the personal computer 12. The spectral sensitivity of the device and the transmission of the optical fiber were then taken into account by software processing.
12
Figure 1. Scheme for measuring the cathodoluminescence spectra. 1 - electron accelerator, 2 - diamond sample, 3 - vacuum chamber, 4 - copper sample holder, 5 - hollow copper heat sink, 6 - capacity with liquid nitrogen, 7 - thermistor, 8 - vacuumtight electric bushings, 9 - ohmmeter, 10 - optical fiber, 11 - spectrometer, 12 - PC.
Thermal contact between sample 2, holder 4, heat sink 5 and thermistor 7 was provided with KPT-8 thermal paste. The temperature of sample 2 was assumed equal to the temperature of thermistor 7 and was calculated from the temperature dependence of its resistance. When measuring the CL spectra of diamond samples, the resistance of the thermistor 7
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varied in the range 232-1085 Ohm, which corresponded to a temperature of 82-295 K. In general, the measurement procedure was similar [6].
3. SPECTRUM ANALYSIS In the work, the CL spectra of nine diamond samples (natural, synthetic HPHT and CVD) were measured. The CL spectra of the samples at 82 K are shown in Fig. 2 a, b and Fig 3. The dashed curves show the digitized spectra of characteristic bands and VSs from references [3, 7-10]. The solid vertical lines indicate the position of the VS ZPL of the NV0 (T1) center at 2.156 eV, the interstitial-related (3H) center at 2.462 eV, the N2V (H3) center at 2.463 eV, the Ni-N (2.56 eV) center at 2.56 eV, the N3V-Ci (N3a) center at 2.68 eV, the N3V (N3) center at 2.985 eV and the interstitialrelated (3.188 eV) center at 3.188 eV. Dotted vertical lines indicate the positions of phonon replicas of ZPL. Phonon replicas were being to the high-energy region from the ZPL that is corresponding to the OA of sample; the phonon replicas were being to the low-energy region from the ZPL that is corresponding to the CL of sample. Samples CN1, CN5, CN9, and CN11 demonstrated similar CL spectra (Fig. 2 a). They contain wide structuralles bands: the green band with maximum at 2.3-2.4 eV, the band-A at 2.85-2.95 eV, and the UV band at 3.5-3.7 eV. The band-A was dominating, and the UV-band had the lowest intensity. Obviously, the CL spectra of the CN1, CN5, CN9, and CN11 samples in Fig. 2 a contained additional components - N3 VS and N3a VS, which will be described below in detail. We note that for the CN5 sample, there was the ZPL of N3 VS in the form of the OA band in the CL spectrum. Wavelength, nm 650 600
550
500
450
400
Wavelength, nm 350
750 700 650 600
82 K
N3
550
450
500
400
2.56 eV
CN11
CN3 N3
CN9 CN5
CN4
CN1
N3a .
Band-A
.
.
. .
2.0
2.5
3.5
3.0
Photon energy, eV
a)
2.0
2.5
3.0
uo
Photon energy, eV
b)
Figure 2 a, b. The cathodoluminescence spectra of diamond samples CN1, CN5, CN9, CN11 (a) and CN3, CN4 (b), measured at cooling by liquid nitrogen. Vertical continuous lines indicate the zero-phonon lines of the T1, H3, 2.56-eV, N3a, and N3 vibronic systems. Vertical dashed lines marked their phonon replicas. The T1, H3, 2.56-eV, and N3 systems are observed also in the form of absorption bands (phonon replica to the short-wave side from zero-phonon lines). Dotted spectra are the result of the digitization of the data given in the handbook [8], including the structureless bands - the green band, the band-A, and the UV band.
In the CL spectrum of the CN3 sample (Fig. 2 b), the green band dominated and the weak band-A and UV band are observed. In addition, there was the 2.56-eV VS with ZPL at 2.56 eV, due to Ni-N centers [8]. The 2.56-eV VS was observed both in the absorption and in the luminescence of CL. In the CL spectrum of the CN4 sample (Fig. 2 b), the green band and the band-A with comparable intensities were displayed, distorted by the H3 VS, the N3a VS and the N3 VS. The intensity of the UV band was low. The CL spectra of the C4 sample were previously reported in Refs [3, 12, 13], which were similar to the CL spectra of the CN1, CN9, and CN11 samples, but with a clearly observed the N3 VS with ZPL at 2.985 eV and its phonon replicas. Figure 3 shows the CL spectra of the C5 sample and the CN8. A common feature was the almost complete absence of the structureless UV band at 3.5-3.7 eV.
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The CL spectrum of the C5 sample showed the green band and the band-A with superimposed the 3H VS with ZPL at 2.462 eV and the 3.188-eV VS with ZPL at 3.188 eV. There is no consensus on the structure of 3H centers and 3.188 eV centers, but we can confidently consider that the centers of these VSs contain interstitial atoms. In addition to CL in the visible range, the C5 sample demonstrated radiative recombination of free excitons with maximum at 5.271 eV [3,13]. The CL spectrum of the CN8 sample (Fig. 3) contained a relatively narrow band at 2.54 eV with the full width at half maximum of 0.2 eV in addition to the green band and the band-A. There was clearly the 3.188-eV VS. Note that samples C5 and CN8 showed the weakest CL (in the visible range) among all nine samples studied. Wavelength, nm 750 700 650 600
550
500
400
450
82 K 3188eV
CN8
C5 ....................... .
......................
.... .........................
:....._
Band -A [12]
Green band [8]
2.0
2.5
3.0
3.5
Photon energy, eV
Figure 3. The cathodoluminescence spectra of the CN8 and the C5 samples under liquid nitrogen cooling. Vertical continuous lines indicate the zero-phonon lines of the 3H, H3, N3a, N3, and 3.188-eV vibronic systems. Vertical dashed lines indicate their phonon replica. In the cathodoluminescence spectra, the H3 and N3 systems were observed also in the form of absorption bands (phonon replicas to the short-wave side of the zero-phonon lines). Dotted spectra are the result of digitization of the data given in the literature [8, 12, 15].
The imposition of various bands and VSs in the CL spectra makes it difficult to analyze the contribution of each components. In the UV-visible range the main broadband components of the CL emission of diamond are the green band, the band-A, the UV band [8, 14], and the 2.5-eV band [8, 15]. These bands can be approximated by Gaussians in the form of: 1 E E0i 2 , I i ( E ) I 0 i exp 2 w
(1)
where Ii (E), I0i is the form factor and amplitude of the i-th spectral component; E, E0i is the photon energy and position of the i-th spectral component and w is the width of the i-th spectral component. Taking into account the contribution of these bands to the CL spectra of the samples it makes possible to determine the CL spectra contribution in pure form of narrow-band components and VSs. Table 2 shows corresponding approximation parameters (1) for all samples. Subtraction of these broadband components from the general CL spectrum yields the difference spectrum that allows analysis of the CL spectrum of the sample on the contribution of different VSs and narrowband components. In this case, in the difference spectrum, VSs and narrowband components were observed both in the form of luminescence CL bands (positive values) and in the form of absorption bands (negative values), since the luminescence of the CL passing through the sample undergoes selfabsorption.
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Table 2. Parameters of Gaussians of the form (1) for approximating the broadband components of the cathodoluminescence spectra of diamond samples. Sample CN1 CN3 CN4 CN5 CN8 CN9 CN11 С4 С5
Imax 366.3 108.9 228.1 68.7 44.1 882.9 145.9 2938 54.4
I01 0.14 0.62 0.18 0.24 0.42 0.13 0.18 0.12 0.24
E01 2.39 2.36 2.3 2.39 2.36 2.39 2.39 2.39 2.33
w1 0.2 0.23 0.2 0.2 0.19 0.21 0.2 0.2 0.18
I02 0 0 0 0 0.51 0 0 0 0
E02 2.54 -
w2 0.09 -
I03 0.93 0.1 0.34 0.72 0.65 0.89 0.88 0.71 0.49
E03 2.88 2.88 2.94 2.88 2.83 2.88 2.85 2.88 2.85
w3 0.25 0.3 0.3 0.26 0.17 0.27 0.25 0.28 0.14
I04 0.05 0.07 0.03 0.03 0 0.04 0.05 0.03 0
E04 3.65 3.5 3.65 3.66 3.65 3.65 3.66 -
w4 0.15 0.25 0.17 0.17 0.2 0.15 0.17 -
Fig. 4 a, b shows the difference emission spectra of the CL of diamond samples obtained after subtraction of broadband components with the parameters from Table 2. Wavelength, nm
Wavelength, nm 600
500
550
450
600
400
82
550
500
400
450
N3
||!
CN11 imiCN9
ALM
.
2o
.
.
.
.
.
2.5
.
.
CN5
:
.
3.0
.
.
.
.
.
.
3.5
2u
Photon energy, eV
a)
.
.
.
.
.
25 3.0 Photon energy, eV
. .
31.5
b)
Figure 4 a, b. Difference cathodoluminescence spectra of the CN1, CN4, CN5, CN9, CN11 samples from Fig. 3 and sample C4 from [13] (a) and the CN3, CN8 and C5 samples from Fig.2 b and Fig. 4a (b). The measurements were carried out with liquid nitrogen cooling. The spectra were purified from the contribution of broadband components to the analysis of the contribution of vibronic systems. The spectral regions of negative values correspond to absorption bands. Vertical continuous lines indicate the zero-phonon lines of the vibronic systems T1, 3H, H3, 2.56 eV, N3a, N3 and 3.188 eV. Vertical dashed lines indicate their phonon replicas.
The difference CL spectra of the CN4, CN1, C4, CN5, CN9, and CN11 samples (Fig. 4 a) contained the N3 and the N3a VSs with ZPL at 2.985 and 2.68 eV, respectively. In all samples the N3a VS was observed only in the CL luminescence, which corresponds to the literature data [8, 16]. The N3 VS was observed both in the form of CL luminescence (2.732.985 eV) and in the form of absorption (2.985-3.35 eV) bands. The ZPL of the N3 VS at 2.985 eV was intensively observed in the CL luminescence of the CN4 and C4 samples, and at the noise level in the CN1 sample spectrum. In the difference spectra of CN5 and CN11 samples, the ZPL of the N3 VS was observed in absorption. In the difference spectrum of the CN9 sample, the ZPL of the N3 VS was not observed neither in the CL luminescence nor in absorption. In the difference CL spectrum of the CN4 sample, the H3 VS was observed at 2.463 eV as an intense CL luminescence band and a weaker absorption band that partially compensated the contribution of the N3a VS. In the difference CL spectra of the CN5 sample, the absorption band of the T1 VS at 2.156 eV was observed. The difference CL spectra of the C5, CN8, and CN3 samples (Fig. 4 b) did not contain the N3 or the N3a VSs. These samples demonstrated the 3.188-eV VS in the CL luminescence. Usually, the 3.188-eV VS was observed in the luminescence emission and practically never in the OA [8].
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The C5 sample was demonstrated the 3H VS at 2.462 eV. Intense phonon replicas were observed, but the ZPL at 2.462 eV did not manifest itself clearly. We also note the observation of anti-Stokes components of the 3H VS (or associated bands), which in some cases were observed in literature [8] (see Fig. 3). The CN3 sample, in addition to the 3.188-eV VS, demonstrated the T1 VS at 2.156 eV in absorption and the 2.56-eV VS both in luminescence and in absorption.
4. IDENTIFICATION OF NATURAL SAMPLES The analysis of the CL spectra of diamond samples allows one to assume their impurity-defective composition with a high probability and, consequently, the method of their synthesis. In the CL spectra of CN1, CN4, CN5, CN9, CN11, and C4 samples demonstrated the presence of broad structureless bands – the green band (maximum at 2.30-2.39 eV), the band-A (2.85-2.94 eV), and the UV band (3.50-3.66 eV). The green band was observed in samples of all types of synthesis with different defect-impurity composition. The maximum position varied from 2.3 to 2.4 eV, which coincides with our results (see Table 2). According to the reference book [8], the green band can be associated with various intrinsic and impurity defects – a non-diamond phase, a substitutional boron, H3+dislocations or N3+interstitial complexes, and polyatomic Ni-N complexes. The band-A was long attributed to distorted/dangling bonds in the dislocation core [12, 14], but later studies established its belonging to sp2-hybridized carbon bonds [17]. The band-A was observed in diamonds of any synthesis methods [1618] with a maximum in the range 2.6-3.0 eV, depending on the superposition of other bands or VSs. For the samples studied in this paper, the position of the band-A maximum locates in the range 2.85-2.94 eV (see Table 2). The UV band was also observed in diamonds of all synthesis methods in the 3.5-3.75 eV region, and, like the green band, can be associated with various structure defects [8] – substitutional boron, nickel-containing complex, intrinsic defect in the dislocation structure. Thus, the observation of the above three structureless CL bands does not allow us to determine the method of synthesis of diamond samples. At the same time, all the samples studied, with the exception of CN3, CN8, and C5, demonstrated the nitrogen-related N3 and N3a VSs in the difference CL spectra. At the same time, the N3 VS was observed not only in CL luminescence, but also in absorption. Nitrogen atoms are part of a large number of known optically active centers in diamond. At HPHT synthesis (including natural), nitrogen is initially embedded in the diamond lattice in the form of substitutional N s atoms dispersed in the sample, forming C-defects [8, 16, 18]. C-defects are responsible for the absorption continuum from 1.7-2.3 eV [14, 16, 18]. If the diamond is subjected to irradiation with high-energy particles (electrons, neutrons, -particles, etc.), then free vacancies (V) and intrinsic interstitial atoms (Ci) are generated in the sample. At temperatures above 500-600 °C, the vacancies become mobile and interact with C-centers to form NV complexes, which, depending on the concentrations of substitutional nitrogen and boron, can be in either a negative or a neutral charge state: (2) N s V NV ( NV 0 ) 0 The NV and NV complexes in the optical spectra are manifested in the form of the NV and the T1 VSs, respectively. The NV VS contains the ZPL at 1.945 eV and phonon replicas in the 1.843-1.714 eV region in the luminescence emission and in the 2.236-1.960 eV region in the absorption. The T1 VS contains the ZPL at 2.156 eV and phonon replicas in the 2.133-1.972 eV region in the luminescence emission and in the 2.199-2.160 eV region in the absorption. If a diamond containing C-defects is subjected to HPHT-annealing at temperatures above 1700 ° C and a stabilizing pressure of 7-9 GPa, then the formation of nitrogen complexes of N 2 or A-defects begins [19]:
Ns Ns N2 ,
(3)
which cause the absorption continuum with photon energy > 3.9 eV. At temperatures above 2200 ° C, A-defects may dissociate, i.e. the reaction is inverse (3). At temperatures from 1100 to 1600 °C the A-centers can interact with vacancies V, forming N2V-complexes or H3centers:
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N 2 V N 2V ,
(4) which are centers of the H3 VS with ZPL at 2.463 eV, phonon replicas in the 2.424-2.238 eV region in the luminescence emission and in the 2.692-2.506 eV region in the absorption. A further temperature increasing in the 2200-2650 ° C region leads to various processes of formation and dissociation of defects in which complexes N4V (B center), N3V (N3 center), (Ci)n (B` center) are formed [7, 19]:
N 2 N 2 N 4V Ci , N 2 N 2V N 4V , N 2 V NV N s ,
(5)
NV N s N 2 N 4V ,
(8)
NV N 2 N3V ,
(9)
(6) (7)
Ci Ci Ci ... Ci n , N 4V
(10)
N 4V Ci n N 3V Ci n N s , deformation
(11)
The N3 center is formed in the reactions (9) and (11), representing the three atoms of substitutional nitrogen in the (111) plane in vicinity of common vacancy, i.e. the N3V complex [14, 16, 18]. The N3 VS contains ZPL at 2.985 eV and phonon replicas in the 2.985-2.591 eV region in the luminescence emission and in the 3.379-2.985 eV region in the absorption. The structure of the energy levels of the N3V center is complex. In particular, there is a forbidden transition at 1.495 eV, which is observed in the absorption in the form of the N1 VS with phonon replicas in the 1.809-1.535 eV region. In reactions (5) - (8), B centers are formed, i.e. vacancy and four nitrogen atoms in adjacent lattice sites - N4V-defect. In the luminescence spectra, B centers appear in the form of several lines (the most intense at 5.253 and 5.372 eV), previously called the N9 VS [20]. Later, the N9 VS was associated with the radiative recombination of excitons bound on N4V complexes [21]. At the formation of B centers in the reaction (5), the intrinsic interstitials Ci appears, which then form B` centersor "platelets" in the reaction (10), extended defects (up to 150 μm) in the (100) plane, consisting of intrinsic interstitial atoms [7, 19]. Condensation of B` centers begins around B centers, therefore "platelets" always contain a random number of N4V complexes. It is known that natural diamonds were often subjected to plastic deformations that led to the slip of carbon layers, which in turn caused the dissociation of N 4V (B) complexes in reaction (11) with the formation of N3V (N3) complexes [22]. The interaction of intrinsic interstitials (Ci), which consist in B` defects, with the N3 centers formed during the dissociation of B centers, leads to a decrease in the energy of the excited state of the N3V complex and to a shift to the long-wave region of the ZPL and the phonon wing of the N3 VS [16]. The N3a VS, observed as a result of this interaction, practically repeats in form the N3 VS. ZPL of the N3a VS is observed at 2.68 eV, phonon replicas in the 2.65-2.35 eV region. The N3a VS is always observed only in the luminescence emission. At the same time, in order to observe the N3a VS in an explicit form, it is necessary to cool the sample with liquid helium [8]. In the laboratory, the above processes occur for tens of minutes. The selection of the conditions for HPHT-annealing makes it possible to form complex polyatomic centers like N3V, N4V, (Ci)n, and their derivatives. However, the complex sequence of reactions (5)(10)(11) under laboratory conditions is practically not realized. Under natural conditions, diamonds were formed during the time interval up to 1 billion years [16]. For such a long time, aggregation of the nitrogen impurity occurred and the formation of complex nitrogen centers took place, including macroscopic B` defects. At the same time, due to the movement of the mantle, the external conditions changed, and the processes of plastic deformation occurred, with the formation of complexes N3V-(Ci)n (N3-B`). Therefore, natural diamonds demonstrate the N3 and the N3a VSs in the CL spectra, while synthetic diamonds do not. Thus, the observation of the N3 and the N3a VSs in the difference CL spectra of CN4, CN1, C4, CN5, CN9, and CN11 samples indicates the natural origin of these samples. The CN3, CN8 and C5 samples are highly likely to be synthetic samples. The presence in the CL spectra of CN3, CN8 samples the 2.56-eV vibronic system and the unstructured band at 2.54 eV, associated with nickel, indicates that these two samples are synthetic, grown at high pressure and high temperature. Due to exciton luminescence at 5.271 eV and the absence of any spectral features of impurity nature, the C5 sample was identified as synthetic, grown by the chemical vapor deposition.
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Undoubtedly, the conclusion made in this paper on the observed N3a VS in the difference CL spectra requires experimental verification on a large sample of various natural samples in combination with measurements of IR absorption spectra. It may establish the applicability limits of this method of identification of natural diamonds. It is also interesting to establish the influence of HPHT annealing of natural diamonds on the N3a VS in the difference CL spectra.
5. CONCLUSION The cathodoluminescence spectra for nine diamond samples at room temperature and cooled with liquid nitrogen were studied. Diamond samples had different synthesis methods, different impurity-defective composition and were characterized by signs of different radiation-thermal processing. As a result, diamond samples demonstrated vibronic systems and unstructured luminescence bands of various impurity-defect and single-polyatomic complexes, among which uniquely established nature have such defects as Ci, NV0, Ni-N, N2V, N3V, N3V-(Ci)n. Among these impurity-defective centers, only the latter has not been reproduced in the laboratory at the moment, so it can be a marker of natural diamonds. Consequently, the observation of the N3a vibronic system with a zero-phonon line at 2.68 eV and phonon replicas in the 2.65-2.35 eV region associated with the complex N3V-(Ci)n (N3V center in the plane structure of the intrinsic Ci interstitial sites) makes it possible to identify natural diamonds. For six diamond samples in the difference cathodoluminescence spectra, the vibronic system N3a was observed, i.e. in spectra purified from structureless luminescence bands at 2.3-2.4, 2.6-3.0, and 3.5-3.75 eV, which are usually observed in diamonds of any synthesis method. For two samples, the 2.56-eV vibronic system and the structureless band at 2.54 eV associated with a nickel impurity were demonstrated and attributed to synthetic diamonds grown at high pressure and high temperature from a batch containing nickel. One of the samples studied was a synthetic diamond grown by the method of chemical vapor deposition, demonstrated radiative recombination of free excitons at 5.271 eV, and a complete absence of spectral features of the impurity nature.
The authors thank Kostyrya I.D., Shulepov M.A. and Sorokin D.A. for assistance in the preparation for the operation of the experimental setup and provide of research and measurements. The work was carried out within the framework of the government task of the IHCE SB RAS on the topic №13.1.3.
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